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[NLL] Add false edges out of infinite loops #47802

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merged 6 commits into from
Feb 9, 2018
Merged

[NLL] Add false edges out of infinite loops #47802

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5ca88ae
Fix comment in ExprKind::LogicalOp
sapphire-arches Jan 22, 2018
bdc37aa
mir: Add TerminatorKind::FalseUnwind
sapphire-arches Jan 25, 2018
ed6a2eb
mir: Add false edge cleanup out of infinite loops
sapphire-arches Jan 26, 2018
eae1a35
mir: Add and fix tests for FalseUnwinds
sapphire-arches Jan 27, 2018
8e0c3f5
[ci skip] Generate false edges from loop_block
sapphire-arches Feb 7, 2018
85dfa9d
Fix tests for MIR loop lowering
sapphire-arches Feb 8, 2018
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mir: Add false edge cleanup out of infinite loops 
Fixes #46036
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@sapphire-arches
sapphire-arches committed Feb 5, 2018
commit ed6a2ebcd6d36f2e0760d419094a653bda984bc2
22 changes: 17 additions & 5 deletions src/librustc_mir/build/expr/into.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -156,11 +156,15 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
//
// If `opt_cond_expr` is `None`, then the graph is somewhat simplified:
//
// [block] --> [loop_block / body_block ] ~~> [body_block_end] [exit_block]
// ^ |
// | |
// +--------------------------+
// [block] --> [loop_block] ~~> [loop_block_end]
// | ^ |
// false link | |
// | +-------------------+
// v
// [cleanup_block]
//
// The false link is required in case something results in
// unwinding through the body.
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this picture looks right, but I'm not sure that the code below is doing that...


let loop_block = this.cfg.start_new_block();
let exit_block = this.cfg.start_new_block();
Expand All @@ -174,6 +178,7 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
move |this| {
// conduct the test, if necessary
let body_block;
let out_terminator;
if let Some(cond_expr) = opt_cond_expr {
let loop_block_end;
let cond = unpack!(
Expand All @@ -187,8 +192,15 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
// we have to do it; this overwrites any `break`-assigned value but it's
// always `()` anyway
this.cfg.push_assign_unit(exit_block, source_info, destination);

out_terminator = TerminatorKind::Goto { target: loop_block };
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it would be nice if we indicated what out_terminator means in terms of the diagram above; I find this a bit hard to decipher (kind of a pre-existing problem :)

} else {
body_block = loop_block;
let diverge_cleanup = this.diverge_cleanup();
out_terminator = TerminatorKind::FalseUnwind {
real_target: loop_block,
unwind: Some(diverge_cleanup)
}
}

// The “return” value of the loop body must always be an unit. We therefore
Expand All @@ -197,7 +209,7 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
// Execute the body, branching back to the test.
let body_block_end = unpack!(this.into(&tmp, body_block, body));
this.cfg.terminate(body_block_end, source_info,
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So, in the new diagram above, body_block_end does not appear -- but I presume it is the end of the loop body? In that case, I still think that this is the wrong place for this terminator. I think we want the false unwind to be the terminator for the loop_block. Something like this:

if let Some(cond_expr) = opt_cond_expr {
    ... // as before
} else {
    body_block = this.cfg.start_new_block();
    let diverge_cleanup = this.diverge_cleanup();
    this.cfg.terminate(
         loop_block,
         TerminatorKind::FalseUnwind { real_target: body_block, unwind: Some(diverge_cleanup) }
    );
}

then there is no need for the out_terminator variable.

In particular I think that will be important for this test case, which would be good to add:

struct Foo { x: &'static u32 }

fn foo() {
    let a = 3;
    let foo = Foo { x: &a }; //~ ERROR E0597
    loop { continue; } // <-- note the continue here =)
}

fn main() { }

TerminatorKind::Goto { target: loop_block });
out_terminator);
}
);
exit_block.unit()
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